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From: Jerome Desquilbet <jDesquilbet@Rational.COM>
Subject: Re: Can you help with an Ada 95 OO Problem
Date: 1997/01/21
Message-ID: <32E4C727.1CFB@Rational.COM>#1/1
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references: <na.9173d8474e.a50140averon@argonet.co.uk>
to: David Wallace <averon@argonet.co.uk>
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organization: Rational Software Corporation in (the) SQY, France
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newsgroups: comp.lang.ada
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Date: 1997-01-21T00:00:00+00:00
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David,

First, some comments about the compilability of the program you posted:

- You have to supply a body for Http.Headers.Debug

- The body Http.Headers.General_Headers.Init does not compile because in

    procedure Init (Obj : access General_Header) is
	Header_Ref : Header_Access := Obj;
    begin
        ...
    end Init;

  "OBJ is not a value of HEADER_ACCESS"

- There's another semantic problem in Http.Headers.General_Headers.Debug

I don't know if I well understood your problem. However, I have built a
program handling something similar (again if I understood well).
I.e. how to initialize an object, starting by initializing its "inner
parent object".
I don't think I would create and initialize objects exactely like that,
but never mind. I hope this example will help you.

Good luck,

  J�r�me.


package Base is

    type Object is tagged private;
    procedure Initialize (A_Base : access Object);
    procedure Debug (A_Base : in Object);

private
    type Object is tagged
	record
	    Data : Integer;
	end record;
end Base;


with Ada.Text_Io;

package body Base is

    Arbitrary_Data : constant Integer := 4;

    procedure Initialize (A_Base : access Object) is
    begin
	Ada.Text_Io.Put_Line ("Initializing a base object... begin");
	A_Base.all.Data := Arbitrary_Data;
	Ada.Text_Io.Put_Line ("Initializing a base object... end");
    end Initialize;

    procedure Debug (A_Base : in Object) is
    begin
	Ada.Text_Io.Put_Line (Integer'Image (A_Base.Data));
    end Debug;

end Base;


with Base;

package Derived is

    type Object is new Base.Object with private;
    procedure Initialize (A_Derived : access Object);
    procedure Debug (A_Derived : in Object);

private
    type Object is new Base.Object with
	record
	    Data : Character;
	end record;
end Derived;


with Ada.Text_Io;

package body Derived is

    Arbitrary_Data : constant Character := 'e';

    procedure Initialize (A_Derived : access Object) is
    begin
	Ada.Text_Io.Put_Line ("Initializing a derived object... begin");
	Base.Initialize (Base.Object (A_Derived.all)'Access);
	A_Derived.all.Data := Arbitrary_Data;
	Ada.Text_Io.Put_Line ("Initializing a derived object... end");
    end Initialize;

    procedure Debug (A_Derived : in Object) is
    begin
	Base.Debug (Base.Object (A_Derived));
	Ada.Text_Io.Put (A_Derived.Data);
	Ada.Text_Io.New_Line;
    end Debug;

end Derived;


with Derived;

procedure Try_Derived is
    D : aliased Derived.Object;
begin
    Derived.Initialize (D'Access);
    Derived.Debug (D);
end Try_Derived;

______________________________________________________________________
Jerome Desquilbet                             jDesquilbet@Rational.COM
 ' ^